Origin of chemical contrast in low-energy electron reflectivity of correlated multivalent oxides: The case of ceria

A combined experimental and theoretical study of the local chemistry of cerium oxide films and islands on Ru(0001) is presented. Based on intensity-voltage low-energy electron microscopy [I(V)-LEEM] and resonant x-ray photoemission spectroscopy, we establish a one-to-one correspondence between the local oxidation state of Ce3+ [cubic Ce2O3(111)] and Ce4+ [cubic CeO2(111)] and their respective spatially resolved I (V) curves. Ab initio scattering theory explains the difference between the I (V) curves in the low-energy range in terms of the k(parallel to) = 0 projected band structure arising from the different structure of the Ce 5d states in fully oxidized and reduced ceria. The theoretical analysis unambiguously attributes the LEEM contrast observed for chemically reduced cerium oxide to a variation in oxidation state on the nanometer scale, which is not present for the as-grown islands.